1. Field of the Invention
The present invention relates to a radio communication system and a distance measuring method.
2. Description of the Related Art
As a system for carrying out radio communication between an in-vehicle communication device and a user-owned radio communication terminal, a system is already known that can locks/unlocks the door of a vehicle without using a mechanical key. According to this system, when a user carrying an electronic key approaches the vehicle, the in-vehicle communication device checks the ID code of the electronic key. When the validity of the electronic key is successfully authenticated as a result of the checking and the electronic key enters a predetermined distance range from the vehicle position, the vehicle can be unlocked without having to insert the mechanical key into the cylinder lock of the vehicle. As an electronic key, though only a terminal conforming to the specification of the in-vehicle communication device is used today, study has been conducted on the use of a user-owned general-purpose radio communication device.
Meanwhile, the relative distance not only between an in-vehicle communication device and a radio communication terminal such as an electronic key but also between any two devices that carry out radio communication with each other is calculated in many cases based on the reception radio wave intensity of the radio signal. For example, Japanese Patent Application Publication No. 2005-328231 (JP 2005-328231 A) discloses a system in which the relative distance is detected by a radio terminal device that wirelessly communicates with a fixed access point. The radio terminal device receives a radio signal sent from the access point, detects its radio wave intensity and, based on the detected reception radio wave intensity, estimates the distance to the access point. In addition, based on the distance estimated in this manner, the radio terminal device determines the radio wave intensity of radio communication via which it sends a signal and, at the same time, gradually increases the level of the determined sending radio wave intensity.
However, in a radio communication system composed of an in-vehicle communication device and a mobile radio communication terminal, the following problem may arise. That is, when the in-vehicle communication device measures the distance by communicating with the mobile radio communication terminal that has the function to make variable the sending radio wave intensity as with the device described in the above document, the relative distance between them may be incorrectly detected. In other words, it is difficult to measure the distance when a general-purpose radio communication terminal is used as a terminal that wirelessly communicates with an in-vehicle communication device.